1. Field of the Invention
The present invention relates generally to using radio signals to estimate separation distances between transmitting and receiving radios, and more particularly, to a system and method of estimating a separation distance between an impulse radio transmitter and an impulse radio receiver based on a signal strength of an impulse signal at the receiver.
2. Related Art
A known time-based system for estimating a separation distance between a transmitter and a receiver translates a transit time of a radio signal transmitted between the transmitter and the receiver into a separation distance estimate. Such a system disadvantageously requires complex transceiver circuitry and/or techniques for time-synchronizing the receiver to the transmitter, and for measuring signal time-of-arrival at the receiver.
A known amplitude-based system includes a transmitter for transmitting a radio signal, such as a continuous wave signal having a relatively narrow band frequency characteristic, and a receiver for receiving the radio signal and for estimating a separation distance between the transmitter and the receiver based on a signal strength or amplitude measurement of the narrow band signal at the receiver. Although less complex and generally less expensive than the time-based system, the narrow band, amplitude-based system disadvantageously suffers from multipath fading processes. An example of such multipath effects is called Rayleigh fading, in which signals from many delayed reflections combine and destructively interfere with one another at an antenna of the receiver. This is especially true of mobile urban environments and within edifices where multipath effects may be so extreme as to cause 6 decibels (dB) or more root-means-square signal strength deviations with more than 30 dB or more signal strength nulls at the receiver, resulting in thirty-to-one (30:1) distance measurement errors. Such large signal strength fluctuations as a function of receiver position lead to inaccurate separation distance estimates and render known amplitude-based systems unusable. Efforts to overcome multipath fading include diversity techniques such as using multiple frequencies and multiple antennas, as well as using spread spectrum coded waveforms. However, such efforts have had only limited success.
Therefore, there exists a need to resist or mitigate the effects of multipath fading in a system for estimating a separation distance between a mobile radio transmitter and a mobile radio receiver based on a signal strength measurement at the receiver.
The present invention is directed to a multipath resistant, impulse radio system for estimating a separation distance between an impulse radio transmitter and an impulse radio receiver. The system includes an impulse transmitter for transmitting an impulse signal having one or more of an ultra-wideband or medium wideband frequency characteristic. The ultra-wideband and medium wideband impulse signal advantageously resists multipath fading effects. An impulse receiver spaced from the transmitter receives the transmitted, multipath resistant impulse signal. The receiver includes a signal evaluator configured to measure a signal strength of the received impulse signal. An estimator, coupled to the evaluator, estimates the separation distance based on the measured signal strength. The present invention improves the accuracy of the separation distance estimate over known systems by utilizing multipath resistance afforded by the impulse radio signal. Accordingly, the separation distance estimation accuracy in the present invention is relatively immune to multipath effects compared to narrow band amplitude methods. As a result, the present invention offers performance improvements over narrow band amplitude based methods and a simpler solution compared to timing based methods.
In one embodiment of the present invention, the receiver receives a plurality of impulse signals including impulse signals arising from multipath effects. The receiver classifies a signal propagational/multipath environment using the received impulse signals, and then selects a radio propagation path loss model corresponding to a classified multipath environment. The estimator translates the measured signal strength to a separation distance based on the selected radio propagation path loss model. In doing so, the receiver further improves the accuracy of the separation distance estimate because the estimate takes into account information regarding the propagation environment, which can be deduced from patterns in received multipath impulse signals.
Further features and advantages of the invention, as well as the structure and operation of various embodiments of the invention, are described in detail below with reference to the accompanying drawings.